This invention relates to an acoustic underwater communication system, and more particularly to an apparatus for transmitting command or control signals to underwater equipment and for receiving data signals therefrom. The invention further relates to a system for detecting an event near the underwater equipment which may forecast an impending disaster, and to a back-up acoustic communications link in a vessel launched in an emergency to dive below any occluding disturbance in the water.
Mineral exploration, development and production equipment operating under water is currently being implemented with either hydraulic or electronic/hydraulic systems which require the use of either electrical cables or hydraulic pilot control cables, extending from the sea-floor structure to a remote control center which may be a drilling platform, a production platform or a centralized subsea control station which, in turn, is controlled remotely from either a surface platform or shore station. Since all of these systems depend upon maintaining the integrity of a "hard wire" in the water, there is increasing concern, both in the industry and in various regulatory agencies, that catastrophic failures may occur if this link parts for any reason, such as may result from a catastrophic blowout at a wellhead, heavy weather, inability of a platform to maintain station with a consequent drive-off, etc. An acoustic communication link that propagates valve control commands directly through the water would provide a method of maintaining control in the event of failure of the hard wire link. In the case of a failure in an oil producing operation (where there is no evidence of damage or leakage in the production flow lines) an acoustic link could be used to reopen the production valves which are currently designed to close automatically in the event of primary control failure, such as power loss, or a hydraulic control line parting.
Other examples of activities and operating conditions under which an acoustic communication link could be used to advantage will be evident. Consequently, it will be apparent that the present invention is not limited to exploration and production of oil. Reference to that type of activity serves merely to illustrate the importance of the invention, particularly in connection with a catastrophic gas blowout occurring during otherwise routine drilling operations. The magnitude and manner of the blowout would be such that primary system control (hydraulic or electrohydraulic, depending on hard-wire communication from the drilling platform to the wellhead structure) is either deliberately or inadvertently disconnected. The present invention would provide an acoustic link that could be used as a secondary command/control link with greater reliability than has heretofore been possible.
In the past, efforts have been made to use acoustic command/control links in underwater wellhead operations, as shown in U.S. Pat. No. 3,750,096. Briefly, valves positioned under water at a wellhead are controlled by coded acoustic transmissions generated at a control station on an offshore rig through an underwater transducer fastened to the rig and hard wired to the control station.
An acoustic communication system has been found to be advantageous not only in transmitting data relating to monitored parameters from underwater equipment to a control platform but also in providing an emergency or back-up link. It is desirable to obtain monitored data upon interrogation, and to be able to check or verify the operation of the underwater communication equipment upon interrogation by confirming critical signal path integrity without actually interrupting operation of equipment being monitored and/or controlled.
A major problem with prior acoustic communication systems is that, at the moment of greatest need, the communications link is very likely to be occluded by great turbulence in the water. If the acoustic transducer is attached to or suspended from the platform rig at a sufficiently great depth, this turbulence (which covers a greater area near the surface of the water in the event of a gas blowout) can be avoided, but this approach requires extending the hard-wire link from the surface over a greater length through the turbulence, thereby increasing the probability that the communications link will be broken through a parting of the hard-wire link. Alternatively, it has been suggested that the acoustic transducer be suspended from a small boat deployed a sufficiently great distance from the platform that a turbulence free communication path exists from the transducer to the underwater control apparatus. Such an approach is, however, unattractive because of the risk involved in launching a small boat, and the valuable time expended in deploying it the sufficient distance.